The present invention relates to a digital magnetic recording and reproducing apparatus, and more particularly to a playback equalizing circuit for improving the quality of a playback signal during reproducing at a variable speed such as a high-speed search in a video cassette recorder, and stably performing playback equalization.
A digital video cassette recorder (hereinafter referred to as a VCR) is an apparatus for converting an analog image signal into a digital image signal, recording the digital signal onto a magnetic tape, and then reproducing the image. The digital data is recorded onto a magnetic tape in the direction of magnetization corresponding to the recording current whose polarity is reversed at the rising and falling edges of a pulse signal. During reproducing, the change of the reproducing current corresponding to the direction of magnetization is detected, and the pulse signal is restored. Also during reproducing, the signal picked up through a playback head is amplified, and then is equalized to be high-frequency-compensated. Then, the original signal and the delayed signal are matched to each other, and its waveform is shaped to be compared with a constant reference voltage, so that the pulse signal is reproduced. A conventional method for reproducing a pulse signal is explained in detail with reference to FIG.1.
A conventional playback equalizing circuit illustrated in FIG. 1 includes a playback head 11 for converting information recorded on a magnetic tape 10 into an electrical signal, an amplifier 12 for amplifying the signal picked up from the playback head, an equalizer 13 for reinforcing the high-frequency component of the playback signal, a delay line 14 for delaying a signal having a reinforced high-frequency component for a predetermined time, an adder 15 for summing the undelayed signal having a reinforced high-frequency component and the signal delayed for a predetermined time, first and second comparators 16 and 18 for respectively comparing first and second reference voltages with the output of the adder, first and second reference voltage generators 17 and 19 for generating the first and second reference voltages, and an OR gate 20 for the logical operation of the outputs from the comparators.
The operation of the playback equalizing circuit constructed as above will be explained in reference to the waveforms of FIG. 2.
FIG. 2A shows the original signal recorded on a magnetic tape. When the digital data recorded on magnetic tape 10 is picked up through playback head 11, the output waveform can be represented as shown in FIG. 2B. The high-frequency component of the output waveform is compensated via equalizer 13, and the waveform then appears as shown with solid lines in FIG. 2C. (Here, a low-pass filter can be installed in order to remove a low-frequency component among the signals output by equalizer 13.) The frequency-compensated signal (represented as solid lines) is delayed through a delay line 14, to become the waveform represented by dotted lines (FIG. 2C). The solid-line waveform output from equalizer 13 and the dotted-line waveform output from delay line 14 are summed in adder 15, thereby eliminating the out-of-phase portions. The result is output as shown in FIG. 2D. The waveform of FIG. 2D is simultaneously supplied to the non-inverting input terminal of a first comparator 16, to be compared with a first reference voltage of a first reference voltage generator 17 which is supplied to the inverting input terminal, and the inverting input of a second comparator 18, to be compared with a second reference voltage of a second reference voltage generator 19. The output waveform of first comparator 16 is represented in FIG. 2E, and FIG. 2F is that of second comparator 18. The OR gate 20 OR-operates the output waveforms of the first and second comparators as shown in FIGS. 2E and 2F, to output a waveform as shown in FIG. 2G.
Also, a differential method illustrated in FIG. 3 is used as another method for extracting such a digital signal, which differentiates the signal reproduced from a head, distinguishes the rising or falling edges of the digital signal by detecting the zero-crossing points of the differentiated signal, and restores the waveform of the digital signal. An apparatus using the differential method includes a differentiator 26 for differentiating the output signal of equalizer 13 without using the delay line 14 of FIG. 1, a zero-crossing detector 27 for detecting the zero-crossing points of the differentiated signal, a voltage comparator 28 for outputting a peak value according to the value resulted by comparing the reference voltage and the output signal of equalizer 13, an AND operator 29 for AND-operating the output signal of voltage comparator 28, and the output signal of zero-crossing detector 27, and a flip-flop 30 for reproducing the original digital signal by means of the output of AND operator 29, thereby reproducing a signal.
The operation of the playback equalizing circuit according to such a conventional differentiating method will be explained in reference to the waveforms of FIG. 4.
FIG. 4A shows the waveform of a recorded digital signal, and FIG. 4B shows the pick-up signal output from a playback head, FIG. 4C shows the signal after its high-frequency component is reinforced by the equalizer 13, and FIG. 4D shows the differentiated signal shown in FIG. 4C. FIG. 4E shows a zero-crossing pulse signal which represents the extracted zero-crossing points of the differentiated signal, which includes pulses e1 through e6 generated at a point where the recorded digital signal corresponds to rising or falling edges, and pulse e7 generated by a similar zero-crossing according to a noise component. FIG. 4F shows a gate signal generated by voltage comparator 28 which compares the output of low-pass filter 25 with the voltage supplied to the reference voltage terminal, and which outputs a peak value. FIG. 4G shows a zero-crossing pulse signal resulting from the AND operation (performed by AND operator 29) of the zero-crossing pulse signal and the gate signal, wherein pulse e7 is removed. This signal drives a flip-flop 30 to obtain a restored digital signal as shown in FIG. 4H.
Such an apparatus for reproducing a signal experiences no problems while reproducing an ordinary signal, but does when reproducing at a variable speed, that is, when a special function is required. Such a problem will be described with reference to FIGS. 5A and 5B.
FIG. 5A is a diagram showing the trace of review or cue operation of a playback head scanning a tape, and FIG. 5B shows envelope waveforms of the picked-up signal of FIG. 5A. As shown in FIG. 5B, the picked-up signal during special reproducing exhibits a remarkable reduction in the amplitude of the front and rear parts of its scanning period as compared with the central portion thereof. Such phenomena is generated because the head traverses and scans the track where signals on a tape are recorded, and is unavoidable in a rotary-magnetic-head-drum-type recording and reproducing apparatus. When such a signal is input to a reproducing apparatus such as those illustrated in FIG. 1 or 3, the amplitude of a high frequency component thereof is unstable, so that the signal is much weaker than the reference voltage. Therefore, the signal is difficult to reproduce, and sometimes cannot be reproduced at all. Accordingly, in the case of a special reproducing, in order to reproduce the recorded signal, it is necessary to lengthen a valid term of a picked-up signal to reproduce the amplitude of the front and rear parts of the picked-up signal which is equal to that of its central part.
Also, since in such a playback equalizing circuit, during reproducing at a variable speed, a playback head traverses and scans a plurality of tracks on a magnetic tape, and the level of a reproduced high-frequency signal becomes unstable, it is difficult to reproduce a signal accurately when compared by means of a constant reference voltage, and thereby renders reproduction impossible. Accordingly, during reproducing at a variable speed, it is necessary to respond to a reproduced signal, control the reference voltage level, and stabilize the playback characteristics.